On modern rock drilling rigs, the positions of the drill booms are measured in order to be able to drill the holes in the correct place in the rock, possibly by using automatic controls. When measuring a position for a drill hole, the angles are measured at each joint of the drill booms and any telescopic movements of these. A boom has normally five to six joints, for which reason the measuring must be carried out with a high degree of precision in order to be able to calculate where the hole will be drilled.
In those instances where gearboxes are used in order to achieve the rotations in the joints of the drill booms, normally the angle in the gearbox is measured by measuring on the input motor shaft, whereby one multiplies with the gear ratio of the gearbox.
The problem with using a sensor assembly which measures the angle in the gearbox by measuring on the input motor shaft of the gearbox is that one does not measure the play which exists in nearly all gearboxes, which means that one, in applications that require a high precision, must use gearboxes which do not have any play, which results in use of expensive gearboxes as standard gearboxes have a play of the order of 0.3 degrees, which cannot be accepted when positioning.